Discussion about torque and power often gives an impression they are quite separate features of an engine’s performance. In reality, power is merely, as they say in the language of physics, a function of torque.

Apart from Einstein’s E = mc², the only other formula I’ve seen on T-shirts is:

Power = Torque x RPM

            5252

This simple equation explains the torque/power relationship beautifully.

What it says is that at any particular engine speed, the power (in horsepower) can be calculated by multiplying the torque in lbs-ft by the engine speed in rpm and then dividing it by the constant, 5252.

To understand torque it’s best to drop the popular notion that it’s the “twisting force” that an engine generates, unless you’re the kind of guy who arm-wrestles his engine’s flywheel to see how it’s going.

Fortunately the back wheel makes it easier for you by converting the “twist” to straight-line force. So when you launch your bike, the thrust, the pull, the tractive force that stretches your arms and pushes you rearwards – the force that you can feel – is torque. Small torque gives small thrust. Big torque gives big thrust. You can’t, however, feel power.

Power is the effect of the acceleration that the torque delivers, over a period of time. Drag racing illustrates this pretty well. In the run down the quarter-mile your sense of the engine’s torque at any instant will come from the strength of the acceleration that you feel. Your sense of the engine’s power will come from your elapsed time for the run. So, it can be said that as a rider you can measure the torque through the seat of your pants, but you’ll need a stopwatch (or a dyno) to measure the power.

And please, never buy a beer for anyone who uses the expression “pulling-power”. Ain’t no such thing. If you’re talking “pull’’, you’re talking “torque”.

HORSEPOWER VS KILOWATTS

Like a lot of our readers and people in the industry we’ve grown up on the “horsepower” thing and despite 40-odd years of learning to live with metrics we’ve clung to horsepower.

Here at MT we’ve been having two-bob each way. We’ve used “hp” figures for peak power while using metrics for peak torque. The time has come to be more consistent. From this point we will use the metric units for torque and power. But as a mark of respect we’ll still give you the “hp” figure in brackets after the “kW” number.

REBELS IN THE RANKS

If you’re like me when you’re bragging about power figures to your mates, quoting kilowatt numbers feels more like you’re talking hot-water services than hot bike engines. We could really dig in our heels and go back to horsepower for everything on bikes. How’s about that 55W headlight globe? Now, at 746W per hp, let me see… Yes, that’s 1/14-horsepower of beam guiding me home…   

BUT WHAT ABOUT PS?

When some of us were about 20 and knew everything – including that “hp” meant horsepower – we were bemused when we read that the new Kawuzyami 250 produced “30ps”. It was in the brochures and even in some road-test spec panels. But no-one could tell us precisely what it meant.

Not being totally stupid we did notice that power ratings in “ps” always gave a slightly bigger number than ratings in “hp”. So we knew that these “ps” things weren’t quite the full horse. So we reckoned it stood for “pony-size”.

The pony label served us well enough until the day of enlightenment when we learned the new and exotic term “pferdestarke”, abbreviated as “ps”.

We were probably a little disappointed to find that it more or less just means horsepower in German. So ps was the “metric” horsepower term – sometimes called DIN horsepower.

Its use has largely disappeared since the general adoption of metric units has brought kilowatts to the fore for power ratings, but for the record: 1ps equals 0.9863hp.